1. Field of the Invention
The present invention relates to an image forming apparatus having plural photoconductors.
2. Description of the Related Art
There has been known an image forming apparatus, i.e., a so-called tandem type image forming apparatus, in which plural toner images are formed by means of plural photoconductors, each corresponding to each toner image, with an electrophotographic process, and these toner images are superimposed. In a tandem type image forming apparatus that forms a full-color image, toner images of respective color components, such as yellow ), magenta (M), cyan (C), and black (K), are formed by means of different photoconductors, and each of the toner images is superimposed (see, for example, Japanese Unexamined Patent Application No. 2005-266425).
In the tandem type image forming apparatus, it is necessary to drive the plural photoconductors, each corresponding to each toner image, and an image forming section for forming toner images onto the corresponding photoconductors. The number of components can be reduced by driving the photoconductors of Y, M, and C, which are simultaneously driven, and the corresponding image forming sections (including a developing unit) with a single motor in order to reduce the number of components in a drive section so as to downsize the apparatus. On the other hand, as for the black color, the K photoconductor and the K image forming section (including a K developing unit) are driven with a motor different from the motor used for the YMC, since the sections involved with the black color solely form an image during the formation of a monochromatic image. A stepping motor can be used, for example, as a motor for driving the photoconductors of the respective colors and the corresponding image forming sections. However, it is preferable to use a DC motor, which has a driving force per volume greater than that of the stepping motor, in order to drive a great number of loads, such as the loads for the YMC, with a single motor.
In a structure in which each of the photoconductors of the respective colors and the corresponding image forming sections are independently driven, there may be a case in which a capacity of the K developing unit is set to be greater than the capacities of the developing units for the other colors in order to make a frequency of an exchange of the K developing unit equal to that of the developing units for the other colors, since the K developing unit is more frequently used for the monochromatic printing than the other colors. In this case, a DC motor having a great driving force is preferable. A DC motor may sometimes be used for the other colors in order to share a control circuit and a control program with K. However, the problems described below arise when the DC motor is used for the drive.
Specifically, each of the photoconductors has a very small eccentricity due to a processing precision or assembling precision of components. This eccentricity produces a speed irregularity, which agrees with the rotating cycle, in a peripheral speed. A banding (periodic occurrence of coarse portions and fine portions) is produced due to the speed irregularity. When the high-density portions (fine portions) and the low-density portions (coarse portions) in the respective toner images are different in case where the toner images having the banding are superimposed, a color misregistration occurs, and this color misregistration is noticeable. In view of this, in order to match the high-density portions and the low-density portions in the respective toner images, the photoconductors are assembled with the rotational phase thereof adjusted. Further, the drive of each of the photoconductors is controlled so as to keep the adjusted rotational phase.
The control of the rotational phase is easy, if a stepping motor is used. However, when a DC motor is used, an increase curve of the speed of each of the YMC photoconductors and an increase curve of the speed of the K photoconductor during a period from when the respective photoconductors are started to when they reach a predetermined process speed might not be matched. This causes either the YMC photoconductors or the K photoconductor to rotate faster. Accordingly, a misregistration occurs in the rotational phases of the YMC photoconductors and the K photoconductor.
This will be described in more detail. FIG. 16 is a waveform chart illustrating the change in the speed when a photoconductor drum is stopped by means of a DC motor used for a drive source in a conventional image forming apparatus. During the image formation, the photoconductor drum rotates with a constant speed Vf. In order to stop the photoconductor drum, the supply of electric current to a motor is discontinued to allow the photoconductor drum to naturally stop, or the motor is operated as an electromagnetic brake to cause a forced brake, by which the photoconductor drum is stopped. This corresponds to a time td in FIG. 16. When the photoconductor drum is naturally stopped, the photoconductor drum rotates for a while by inertia due to inertial load even after the supply of electric current to the motor is discontinued.
Compared to a deceleration-change characteristic curve (A1K) of the motor that drives the K photoconductor, a deceleration-change characteristic curve (A1CL) of the motor that drives the Y, M, and C photoconductors has a gentle slope. This is because the motor driving the K photoconductor has a reduced load compared to the motor driving the Y, M, and C photoconductors. When the deceleration-change characteristics of both motors are different from each other, the misregistration in the phases occurs. When the photoconductors are stopped by a forced brake, a time when the photoconductors rotate with inertia is shorter compared to a case in which the photoconductors naturally stop. Specifically, a slope of each of the deceleration-change characteristic curves is sharper than the slope thereof in the case of natural stopping. Even so, each of the photoconductor drums rotates with inertia for a while. In this case, the slope of the deceleration-change characteristic curve (A2CL) of the motor driving the Y, M, and C photoconductors is gentler than the slope of the deceleration-change characteristic curve (A2K) of the motor driving the K photoconductor. In the case of the forced brake, the misregistration in the phases occurs due to a difference in the deceleration-change characteristics.
In order to prevent the misregistration in the rotational phases during the deceleration, there has been proposed an apparatus in which each of the photoconductors is driven to rotate for a predetermined time with a second revolution, which is slower than a first revolution that is the revolution during the image formation, and then, each of the photoconductors is stopped, when each of the photoconductors is to be stopped (see, for example, Japanese Unexamined Patent Application No. 2005-266425).
However, even when the stopping control described in Japanese Unexamined Patent Application No. 2005-266425 is executed, the misregistration in the rotational phases becomes non-negligible even by executing the stopping control described above, when a difference in the loads of the motors is great. This is unfavorable from a viewpoint of preventing the color misregistration.